Using units
Standard units
Scientists use Standard International (SI) units. The table shows some common units.
| Quantity | Unit name | Unit symbol |
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | degree Celsius | °C |
| Pressure | pascal | Pa |
| Energy | joule | J |
| Quantity | Length |
|---|---|
| Unit name | metre |
| Unit symbol | m |
| Quantity | Mass |
|---|---|
| Unit name | kilogram |
| Unit symbol | kg |
| Quantity | Time |
|---|---|
| Unit name | second |
| Unit symbol | s |
| Quantity | Temperature |
|---|---|
| Unit name | degree Celsius |
| Unit symbol | °C |
| Quantity | Pressure |
|---|---|
| Unit name | pascal |
| Unit symbol | Pa |
| Quantity | Energy |
|---|---|
| Unit name | joule |
| Unit symbol | J |
Prefixes for units
Scientists measure a wide range of quantities, some very large and some very small. Measurements for different An order of magnitude estimate approximates a number to the nearest power of ten. have specific prefixes:
| Prefix name | Prefix symbol | Factor | Power |
| tera | T | 1,000,000,000,000 | × 1012 |
| giga | G | 1,000,000,000 | ×109 |
| mega | M | 1,000,000 | ×106 |
| kilo | k | 1,000 | × 103 |
| centi | c | 0.01 | ×10-2 |
| milli | m | 0.001 | ×10-3 |
| micro | μ | 0.000,001 | ×10-6 |
| nano | n | 0.000,000,001 | × 10-9 |
| Prefix name | tera |
|---|---|
| Prefix symbol | T |
| Factor | 1,000,000,000,000 |
| Power | × 1012 |
| Prefix name | giga |
|---|---|
| Prefix symbol | G |
| Factor | 1,000,000,000 |
| Power | ×109 |
| Prefix name | mega |
|---|---|
| Prefix symbol | M |
| Factor | 1,000,000 |
| Power | ×106 |
| Prefix name | kilo |
|---|---|
| Prefix symbol | k |
| Factor | 1,000 |
| Power | × 103 |
| Prefix name | centi |
|---|---|
| Prefix symbol | c |
| Factor | 0.01 |
| Power | ×10-2 |
| Prefix name | milli |
|---|---|
| Prefix symbol | m |
| Factor | 0.001 |
| Power | ×10-3 |
| Prefix name | micro |
|---|---|
| Prefix symbol | μ |
| Factor | 0.000,001 |
| Power | ×10-6 |
| Prefix name | nano |
|---|---|
| Prefix symbol | n |
| Factor | 0.000,000,001 |
| Power | × 10-9 |
Chemists also use the prefix deci (d, power 10-1) when discussing volume:
- 1 dm = 10 cm3
- 1 dm3 = 10 × 10 × 10 = 1,000 cm3
Converting units
To convert between a unit with difference prefixes, multiplication or division of the right factor of ten (10, 100, 1000) is used.
Always do a sense check when multiplying or dividing and ask "is the answer expected to be larger or smaller?"
Example
Question
Convert 5 cm into mm.
1 cm = 10 mm so the conversion is × 10
5 × 10 = 50 mm
This is what would be expected because millimetres are smaller than centimetres.
Time is dealt with differently if it is recorded in minutes, as this is not an SI unit:
- 1 minute = 60 s
Example
Question
Convert 2 minutes 40 s into seconds.
2 minutes = 2 × 60 = 120 s
2 minutes 40 s = 120 + 40
= 160 s
